Carbon dioxide (CO2) is a gas produced as a by-product in large quantities in certain industrial operations, e.g. the manufacture of ammonia. Release of this by-product into the atmosphere is undesirable environmentally as it is a greenhouse gas. Much effort has thus been made towards the development of techniques for the disposal of CO2 in a way other than simple release to the atmosphere. One technique of particular interest is to pump the CO2 into porous sub-surface strata (i.e. lock), e.g. down an injector well in an oil field.
Subsurface disposal can be simply into porous strata or beneficial advantage of the subsurface disposal can be realised if the stratum into which it is disposed is hydrocarbon-bearing as the injected CO2 serves to drive hydrocarbon (e.g. oil or gas) in the stratum towards the producer wells (i.e. wells from which hydrocarbon is extracted). Injection of CO2 is thus one standard technique in late stage reservoir management for achieving enhanced recovery of hydrocarbons.
The quantities of carbon dioxide involved when disposal is by subsurface injection are immense, generally of the order of millions of tonnes. This poses problems in terms of transporting the CO2 from the site at which it is created to the site at which it is injected, especially where the injection site is offshore. Carbon dioxide at ambient temperatures and pressures is gaseous and, if transported batchwise, such voluminous containers are required that the process would be unfeasible. While transport by pipeline might in some circumstances be feasible, the required infrastructure is expensive. It is therefore desirable to transport the carbon dioxide, especially to offshore injection sites, batchwise in liquid form.
Transport of liquid carbon dioxide is however not a problem- or expense-free exercise. If the liquid CO2 is not refrigerated, the pressures required to maintain it in the liquid state are high (60-70 bar) making the required wall thicknesses of the pressurized containers high and making such containers for large scale unrefrigerated liquid CO2 transportation immensely expensive. Transport of liquid CO2 at sub-ambient temperatures reduces the required pressures and required container wall thicknesses but is expensive since refrigeration is required and, as carbon dioxide has a solid phase, there is a risk that solid carbon dioxide can form. Solid carbon dioxide formation makes CO2 transfer by pumping problematic and, due to the risks of pipe or valve blockage, potentially dangerous.
Thus in balancing the economies of refrigeration and container cost and avoiding the risk of solid CO2 formation, in any given circumstances there will generally be a temperature and pressure which is optimal for the liquid CO2 in the containers, e.g. a temperature which is below ambient and a pressure which is above ambient but still sub-critical (the critical point of CO2 is 73.8 bar A). Typically for large scale liquid O2 transport the optimum temperature is likely to be in the range −55 to −48° C. and the pressure is likely to be 5.5 to 7.5 bar A, i.e. corresponding to the position in the phase diagram for CO2 which is just above the triple point in terms of temperature and pressure. The triple point for CO2 is 5.2 bar and −56.6° C. Higher pressures require more expensive containers; and lower pressures and temperatures raise the risk of solid formation.
The type of pressurised containers used on water-going vessels for transport of liquefied petroleum gas (LPG) from a producer well-head to shore are not generally suitable for transport of LCD since the pressures required for LCD transport are higher. Moreover, the liquid transfer apparatus for transferring LPG from an offshore producer well to the water-going vessel (i.e. ship) are unsuitable for transfer of LCD from the vessel to an injector well-head since LPG does not pose the risk of solids formation that are encountered with LCD.